1. Field of the Invention
The present invention relates to an improvement in a structure of a vibration wave motor driven by a travelling vibration wave.
2. Description of the Prior Art
As shown in the U.S. Pat. No. 4,019,073, a vibration wave motion transduces a vibration motor created in an electrostrictive element, when a periodic voltage is applied thereto, to a rotating or linear motion. Since it requires no winding, unlike a conventional electric motor, it is simple in structure and compact, presents a high torque at a low rotating speed and has a low inertia moment.
However, in the known vibration wave motor, in transducing the vibration motion to the rotating motion, a movable member such as a rotor which contacts a vibration motor is unidirectionally friction-driven by a standing vibration wave generated in the vibration member. Therefore, the movable member friction-contacts the vibration member in a forward motion of the vibration and is separated from the vibration member in a reverse motion of the vibration. As a result, the vibration member and the movable member must be in contact with each other within a small distance, that is, essentially in a point or line contact. Thus, the friction-drive efficiency is low.
Since a drive force acts in only a given direction, the movement of the movable member is unidirectional. In order to move the movable member reversely, it is necessary to mechanically switch the direction of vibration by another vibration member. Thus, in order to provide a reversibly rotating vibration wave motor, a complex device is necessary. This reduces the advantages of the vibration wave motor, that is, simple structure and compactness.
In order to resolve the above problem, a vibration wave motor driven by a travelling vibration wave has recently been proposed.
FIG. 1 shows a developed view of such a vibration wave motor.
A vibration absorber 4, a metal ring vibration member 2 having an electrostrictive element 3 bonded thereon and a movable member 1 are inserted, in this order, to a central cylinder 5a of a stator 5 which serves as a base. The stator 5, the absorber 4 and the vibration member 2 are mounted such that they do not rotate with respect to each other. The movable member 1 is press-contacted to the vibration member 2 by a gravity or biasing means, not shown, to maintain the integrity of the motor. A plurality of electrostrictive elements 3a are arranged at a pitch of one half of a wavelength .lambda. of a vibration wave and a plurality of electrostrictive elements 3b are also arranged at a pitch of .lambda./2. The plurality of electrostrictive elements 3 may be a single ring-shaped element polarized at the pitch of .lambda./2. The electrostrictive elements 3a and 3b are phase-differentially arranged at a mutual pitch of (n.sub.o +1/4).lambda.where n.sub.o =0, 1, 2, 3, . . . . Lead wires 7a are connected to the respective electrostrictive elements 3a and lead wires 7b are connected to the respective electrostrictive elements 3b, and the lead wires 7a and 7b are connected to an AC power supply 6a and a 90.degree. phase shifter 6b, respectively (see FIG. 2). A lead wire 7c is connected to the metal vibration member 2 and it is also connected to the AC power supply 6 a.
A friction area 1a of the movable member 1 is made of hard rubber to increase the friction force and reduce abrasion and it is press-contacted to the vibration member 2.
FIG. 2 illustrates the generation of the vibration wave in the motor. While the electrostrictive elements 3a and 3b bonded to the metal vibration member 2 are shown adjacent to each other for the sake of convenience of explanation, they meet the requirement of the .lambda./4 phase shift and are essentially equivalent to the arrangement of the electrostrictive elements 3a and 3b of the motor shown in FIG. 1. Symbols .sym. in the electrostrictive elements 3a and 3b indicate that the electrostrictive elements expand in a positive cycle of the AC voltage, and symbols .crclbar. indicate that they shrink in the positive cycle.
The metal vibration member 2 is used as one of the electrodes for the electrostrictive elements 3a and 3b, an AC voltage of V=Vo sin .omega.t is applied to the electrostrictive elements 3a from the AC voltage supply 6a, and a voltage of V=Vo sin (.omega.t.+-..pi./2) which is phase-shifted by .lambda./4 is applied to the electrostrictive elements 3b from the AC power supply 6a through the 90.degree. phase shifter 6b. A sign "+" or "-" in the above equation is selected by the phase shifter 6b depending on the direction of movement of the movable member 1 (not shown in FIG. 2). When the sign "+" is selected, the phase is shifted by +90.degree. and the movable member 1 is moved forwardly, and when the sign "-" is selected the phase is shifted by -90.degree. and the movable member 1 is moved reversely. Let us assume that the sign "-" is selected and the voltage of V=Vo sin (.omega.t-.pi./2) is applied to the electrostrictive elements 3b. When only the electrostrictive elements 3a are vibrated by the voltage of V=Vo sin .omega.t, a standing vibration wave shown in FIG. 2(a) is generated, and when only the electrostrictive elements 3b are vibrated by the voltage of V=Vo sin (.omega.t-.pi./2), a standing vibration wave as shown in FIG. 2(b) is generated. When the two voltages having the phase difference therebetween are simultaneously applied to the electrostrictive elements 3a and 3b, the vibration wave travels. FIG. 2(c) shows a waveform at a time t=2n.pi./.omega., FIG. 2(d) shows a waveform at a time t=.pi./2.omega.+2n.pi./.omega., FIG. 2(e) shows a waveform at a time t=.pi./.omega.+2n.pi./.omega. and FIG. 2(f) shows a waveform at a time t=3.pi./2.omega.+2n.pi./.omega.. A wavefront of the vibration wave travels in an x-direction.
Such a travelling vibration wave includes a longitudinal wave and a lateral wave. Noting a mass point A of the vibration member 2 shown in FIG. 3, it makes a clockwise rotating elliptic motion by a longitudinal amplitude u and a lateral amplitude w. Since the movable member 1 is press-contacted to the surface of the vibration member 2 and it contacts to only an apex of the vibration plane, it is driven by component of the longitudinal amplitude u of the eliptic motion of mass points A, A', . . . at the apexes so that the movable member 1 is moved in a direction of an arrow N.
When the phase is shifted by +90.degree. by the 90.degree. phase shifter, the vibration wave travels in a -x direction and the movable member 1 is moved oppositely to the direction N.
Thus, the vibration wave motor driven by the travelling vibration wave can switch the forward and reverse rotations with a very simple construction.
A velocity of the mass point A at the apex is represented by V=2.pi.fu (where f is a vibration frequency) and a velocity of the movable member 1 depends on it and also depends on the lateral amplitude w because of the friction drive by the press-contact. Thus, the velocity of the movable member 1 is proportional to the magnitude of the elliptic motion of the mass point A and the magnitude of the elliptic motion is proportional to the voltage applied to the electrostrictive elements.
Since the movable member 1 is friction driven at the apex of the wavefront of the travelling vibration wave of the vibration member 2, it is necessary that the wavefront in the direction of apex (z-axis direction in FIG. 3) resonates in order to improve the drive efficiency. A circumference of the vibration member 2 which is vibrated by the frequency f (=2.pi..omega.) of the input voltage resonates when the circumference is a natural number multiple of a vibration wavelength .lambda..sub.1. That is, it resonates when EQU D=n.lambda..sub.1 /.pi.
where D is a diameter of the vibration member 2.
Thus, the resonation in the z-axis direction at the wavelength .lambda..sub.1 is limited within a small range of the diameter D, and when the ring width of the vibration member 2 is large, the drive efficiency is lowered.